1. Field of the Invention
The invention relates to the field of measurement methods and devices designed to check the integrity of a transmission, especially the integrity of the transmission of a signal transmitted by a radio beacon.
It is known that a radio beacon is a radio device transmitting a set of signals on a carrier frequency that is generally a VHF or UHF frequency. These signals are transmitted in such a way that a moving body, provided with an appropriate receiver tuned to the frequency of the carrier, can use this signal to determine the angular position of the axis, coming from the transmitter, on which this moving body is positioned. Radio beacons may be omnidirectional or directional. Among omnidirectional radio beacons, there is the radio beacon internationally known as the VOR (VHF Omnidirectional Range) beacon that enables an appropriately equipped aircraft to make an ad hoc determination of the angle of the axis, coming from the radio beacon, on which this aircract is positioned in relation to the magnetic north at the position of the radio beacon. Directional radio beacons include, for example, the devices internationally known as ILS (Instrument Landing System) devices. These radio beacons can be used for the determination, in space, of a glide path towards a runway. The signals transmitted by the radio beacons are generally transmitted by an array of antennas, each antenna of the array being powered by a H.F. signal that is modulated in a specific way.
2. Description of the Prior Art
The exactness of the information on a position that may be got from the radio beacons directly depends on the ability of these instruments to transmit a signal that remains within determined norms. In the case of radio beacons such as those of the VOR or ILS systems, these standards are defined by the International Civil Aviation Organization (ICAO). Thus, for example, chapter 3.1. of the document "Appendix 10", published by the ICAO, is devoted to the technical specifications of the ILS.
According to &3.1., the I.L.S. includes the following basic elements:
a) Localizer radio beacon PA1 b) Glide path radio beacon PA1 c) Radio markers
The localizer radio beacon defines a vertical plane, which is the locus of the zero DDM points and contains the runway axis.
The glide path radio beacon defines a surface, the locus of the zero DDM points, whose intersection with the plane of the localizer defines the guidance path.
The radio markers give positional indications with respect to the runway threshold.
The abbreviation DDM designates the difference between the depths of modulation of the carrier by two low-frequency 90 Hz and 150 Hz voltages. The DDM constitutes the guidance signal exploited by the aircraft.
Apart from the performance characteristics themselves, the main requirements relate to the following two points:
continuity of operation: a quality wherein the radiated signal is seldom interrupted;
integrity of the I.L.S.: a quality related to the reliance that can be placed on the exactness of the information given by the installation.
In the I.C.A.O. document, &.3.1.2.6. states that the essential purpose of an ILS installation is to set up and maintain the highest possible degree of integrity, dependability and operating stability of the system under the most unfavorable environmental conditions that might be encountered. The supplement C to the first part contains indications to enable this aim to be achieved.
2.8.2.13 states that, as a rule, the designing of the control equipment relies on the principle of a continuous check of the electromagnetic signals radiated to specific points within the coverage volume, this check making it possible to ensure that they comply with the standards laid down in 3.1.3.11 and 3.1.5.7.
To comply with these directives, the device for checking the integrity of the glide path signal comprises, firstly, a sensor placed in the vicinity of the transmission mast at a position representing the signal transmitted on the glide path and, secondly, a device constituted by couplers measuring the antenna currents and recombining them so as to reconstitute the signal theoretically received in two other directions. With respect to the VOR, the device has at least one sensor located in the vicinity of the antenna array.
The results of the measurements made with these devices should remain within predetermined brackets. The chief drawback of this mode of checking integrity is pointed out by the ICAO, again in appendix 10 on the ILS.
The document states that although this check indicates, to a certain extent, that the electromagnetic signal anywhere else in the volume of coverage is also within the limits of tolerance, this observation is largely got through induction.
It is therefore indispensable to carry out periodic and meticulous checks in flight and on the ground in order to ensure the integrity of the electromagnetic signal throughout the volume of coverage.
For, the field in the vicinity of the onboard receiver is disturbed by all the parasitic reflections coming from the near or distant environment.
This means that the value of the field at a given point depends on the value of the fields radiated in all the other directions and any modification of the antenna patterns is liable to modify the guidance signal at the point of use.
Herein lies the main obstacle to guaranteeing the integrity of the signal in space and the main shortcoming of presently used checking devices. These devices actually measure only representative samples of the signal transmitted in two or three pre-determined directions. While these devices are capable of taking account, in a comprehensive way, of most of the parameters, they can be used neither to make a precise evaluation of the respective values of these parameters nor to determine the field transmitted in every direction of space.